1. Field of the Invention
The present invention is in the area of lubrication of rolling/sliding surfaces that may be required to operate over a large range of kinematic, stress and environmental conditions. Lubrication is conventionally supplied by liquid, solid, and sometimes gaseous materials which provide some kind of low shear strength film between the moving surfaces. The most reliable lubricating films are those generated by hydrodynamic (HD) or elastohydrodynamic (EHD) mechanisms which separate the surfaces by a pumping action due to the convergent nature of contact geometries in motion which generates sufficient pressure to cause a separation of the surfaces. This is done by fluids or lubricants having certain viscous properties. The oils are supplied to the contacting surfaces as a liquid in quantities generally sufficient for lubrication and cooling. Cooling is necessary because of temperature limitations which are almost always related to the oil's viscosity for lubrication or the thermal breakdown of the oil itself. To maintain proper cooling along with lubrication, oils are usually recirculated through a lubrication system with pumps, coolers, sumps, delivery lines and filters. One oil is used with selected viscous properties to cover a specific, but limited, range of temperatures.
This invention provides the processes and devices to lubricate surfaces over a broad temperature range with a small quantity of fluid that can be supplied with a simple system. The invention consists of the lubrication of surfaces with condensed vapors (vapor/condensation, or V/C lubrication) or with vapors that react with the surfaces (vapor/deposition, or V/D lubrication). The condensed vapors are sufficient in thickness to allow EHD film generation and even HD film generation. The amount of lubricant necessary to generate EHD lubricant films is very small, being only a "whisper" of condensate. A companion patent application Ser. No. 08/243,112, filed May 16, 1994, discloses the method for broad temperature range vapor lubrication. Vapors that react with the surfaces (see U.S. Pat. No. 3,978,908 September 1976 E. E. Klaus et al.) are used here as boundary films to supplement V/C lubrication. The preferred mechanism of lubrication is EHD where the surfaces are completely separated with an oil film so that the longest possible life of the surface can be achieved. The boundary film mechanism is called upon when the EHD mechanisms can no longer be maintained.
The invention is most practical with fluids that can be vaporized without leaving breakdown deposits or having the potential for fire. Presently, the most useful fluids are the perfluoropolyalkyethers (PFPE) which do not have a flash or fire point and can be vaporized and condensed without solid breakdown products. Another advantage of PFPE's is their availability over a large range of molecular weights or viscosities. This allows a new form of lubrication which is called variable property (VP) lubrication (see companion patent application Ser. No. 08/243,112). VP lubrication provides a condensate film on the surfaces which increases in molecular weight, or viscosity, with temperature so that HD or EHD lubrication is sustained to an extended temperature level. Chemical additives which are normally insoluble in PFPE's can be supplied as vapors to provide boundary lubrication. The introduction of the additive as a vapor circumvents the solubility limit problem with PFPE's.
2. Description of Prior Art
Most mechanical components are lubricated with systems which:
provide pressurized jets of oil; PA1 drip feed oil; PA1 provide an air-oil mist; PA1 provide oil from oil wicks; or PA1 provide oil from an oil sump into which the components dip as they rotate.
Prior art of this invention is illustrated by example with a gas turbine engine lubrication system, as shown in FIG. 1. A lubrication system of some 30 qts of oil is used to lubricate and cool mainshaft bearings and the accessory gearboxes. An extensive network of high pressure oil feed lines are used with oil jets at the main bearing sites to introduce lubrication and cooling flows. The oil is scavenged and returned to a tank, oil cooler and filter for recirculation.
The technical limits of performance in a gas turbine engine, as well as other engine types, is the temperature limit of the lubricating oil. Extensive efforts are made to prevent oil coking, especially in the feed lines which can become clogged. To prevent coking the oil is cooled and not allowed to become stagnant, especially on certain metals which can catalytically accelerate the coking process. Conventional lubricants are carefully formulated synthetic ester base oils which have good thermal and oxidatively stability. These oils generally fall under the MIL-L-23699 specification which have strict limits on viscosity, pour point, flash temperature, load carrying capacity and coking attributes. The selection of these specifications are to assure that the oil is not too viscous (or solid) for low temperature starting (-40.degree. F.), but yet viscous enough for high temperature running (bulk oil temperature of 450.degree. F.). With current basestock and additive technology, there is little room for improvement with ester base oils. Alternative basestocks are PFPE's and polyphenyl ether (PPE) lubricants. Both of these have inferior boundary lubricating attributes and the latter oil does not have good low temperature properties. To satisfy high temperature operation for military engines high temperature solid lubricants have been investigated, but their performance is not reliable and their life is too limited to be practical. The primary limitations of conventional liquid lubricating systems is their limited temperature range and the complex hardware that is necessary to obtain maximum use out of the performance limits.